A decade of good fortune has run out for Arsenal. Arsène Wenger points with pride to the fact that his team have reached the knockout rounds of the Champions League in every season since Uefa abolished the second group stage 10 years ago but their route has never been as arduous as it is this term.

Even Arsenal's Invincibles did not have it this tough. The class of 2003-04 were the last Arsenal side to really struggle in the group stages but, after gaining just one point from their first three matches, they rallied to win their last three, finishing above Internazionale, Dynamo Kyiv and Lokomotiv Moscow.

Since then the group draws have pitted Arsenal against at least one duff side and their passage to the do-or-die rounds has been relatively secure. This season there are no such things as a gimme: Marseille, Borussia Dortmund and Napoli are all capable of finishing above the Londoners.

Arsenal made a good start by returning from their opening game in Marseille with three points but Tuesday's opponents, Napoli, made an even more impressive one, beating last year's runners-up, Dortmund.

"For Napoli to beat Dortmund is certainly something very special and it will lift their confidence," acknowledged Wenger, who was given an early warning of the threat posed by the Italians during a pre-season game at the Emirates, when Napoli hurtled into a 2-0 lead before two late goals salvaged a draw for the hosts.

Both teams have strengthened since then, Arsenal signing Mesut Özil and Napoli completing a summer revamp under Rafael Benítez in which they reinvested the £55m they received from Paris Saint-Germain for Edinson Cavani to bring in eight new players.

One of their recruits, Gonzalo Higuaín, had been on the verge of joining Arsenal before Real Madrid moved the goalposts and sold the striker to Napoli instead. Wenger knows that containing the Argentinian is one of the major challenges facing the Arsenal's defence but notes that, with Özil around, Napoli will also have to keep a close eye on a former Real player.

Arsène Wenger looks on during an Arsenal training session ahead of their Champions League Group F match against Napoli. Photograph: Paul Gilham/Getty Images

"In the end Real Madrid sold two great players, one of them went to Napoli, one went to Arsenal so it's 1-1 on that front!"

Napoli bought two other players from Real – Raúl Abiol and José Callejón – as well as the exciting Belgian winger Drei Mertens, but Wenger is especially wary of an Italian youngster who was already on their books. Lorenzo Insigne tormented England Under-21s in the summer's European Championship and has begun this season in similar style.

"Insigne is mobile, he is not tall but quick and very good in the transition from defence to attack and he is a good dribbler as well and is an intelligent mover," says Wenger. "That makes him an interesting prospect for Napoli, and of course a dangerous player for us on Tuesday night."

The natural instinct of Wenger's team is to attack but the managers know they must show equal zeal in defence on Tuesday if they are to emerge from such a finely-poised group. "When they win the ball they come out very quickly with Callejón, [Marek] Hamsik, Insigne and Higuaín – they all come out like bombs every time they win the ball. This means that the transition from our team from offence to defence will have to be very quick.

"We have started very well with the win in Marseille, but you know if you win the second game at home you have a good chance to qualify. I wouldn't say it's already a decisive game but it's a game that will have a great weight on the end result of the qualifying teams. Let's deal well with that situation then we will be in a very positive mood to finish the job."